The present invention relates to a process and an apparatus for measuring the effective diameter of the guided mode in a monomode optical fibre. It more particularly applies to the determination of the characteristics of the monomode optical fibres and therefore to optical telecommunications.
It is known that the effective diameter, designated 2w.sub.o, governs the losses resulting from microcurvatures and connections in the case of monomode optical fibres and that the knowledge of this effective diameter as a function of the wavelength makes it possible, for a monomode optical fibre, to determine the diameter of the core and the difference in the optical indices of the step fibre (or jump) with an equivalent index and consequently the losses by curvature and the pass band of said monomode fibre.
Methods for measuring the index profile or radiation pattern in the remote field are known, which make it possible to calculate the parameters of the step fibre with an index equivalent to the monomode optical fibre and from this it is possible to deduce the effective diameter 2w.sub.o of the latter.
However, the index profile measuring methods are very difficult to perform, due to the very small diameter of the core of a monomode optical fibre which is generally less than 10 .mu.m. Moreover, the parameters of the equivalent fibre are then obtained indirectly by calculations and the value of the effective diameter 2w.sub.o, itself obtained by a further and not very accurate calculation.
The methods for measuring the radiation pattern in the remote field require very bright lighting sources, because they involve the measurement of secondary lobes, which have a very low relative intensity. The sources which can be used are lasers of the krypton, argon, helium-neon or YAG-Nd.sup.3+ types, so that the studied fibre must be of the monomode type at their emission wavelength, which varies from 0.5 to 0.8 .mu.m for the first three types and 1.06 .mu.m for the last type, which is not necessarily the case.
The aforementioned methods only permit an indirect determination of the effective diameter 2w.sub.o. A method for the direct measurement of the latter is given in an article by C. A. MILLAR, published in Electronics Letters, vol. 17, no. 13, June 1981, pp. 458-460. However, this method necessitates a special measuring bench for its performance, which imposes a supplementary manipulation of the studied fibre compared with the other measurements which it is desired to carry out thereon. This also applies with regards to the other aforementioned indirect determination methods.